1. Field of the Invention
The present invention relates to a magnetic pole position detector for detecting a magnetic pole position of a synchronous motor when the synchronous motor is started.
2. Description of the Related Art
In synchronous motors including a permanent magnet, a current needs to be fed to the windings of each phase according to the magnetic pole position of the rotor, to produce desired torque. To enable this, the permanent-magnet synchronous motors are controlled so as to feed current to a proper excitation phase (magnetic pole position) by detecting the magnetic pole position of the rotor with a sensor such as an encoder. FIG. 7A and FIG. 7B are schematic diagrams that illustrate detection of a magnetic pole position of a permanent-magnet synchronous motor. A synchronous motor 100 illustrated in FIG. 7A includes a permanent magnet 101 on a rotor, and armature windings 102U, 102V, and 102W on a stator. Note that FIG. 7A does not specifically illustrate the number of times or the way in which the wire of each armature winding is wound, but illustrates a typical example of a permanent-magnet synchronous motor using windings for producing a three-phase rotating magnetic field, the phases being a U phase, a V phase, and a W phase, for example. In addition, FIG. 7A does not specifically illustrate the shape or the arrangement of the permanent magnet, but simply illustrates the permanent magnet as one having the N-pole and the S-pole. When the rotor is in the position illustrated in FIG. 7A, the rotor is drawn on a dq coordinate plane as illustrated in FIG. 7B. For example, assume that a magnetic pole of the rotor 101 of the synchronous motor 100 is at the position illustrated in FIG. 7B (referred to as an “initial magnetic pole position” hereinafter), i.e., the position shifted by the angle δ from the d-axis, when the synchronous motor 100 is started. In this case, data detected by a sensor is corrected on the basis of the initial magnetic pole position, and the rotation of the rotor is controlled by controlling the current phase (excitation phase) of each of the armature windings 102U, 102V, and 102W on the basis of the corrected data.
When the initial magnetic pole position used for correcting data detected by the sensor is inaccurate, the rotation of the synchronous motor is controlled by using the inaccurate data. This often leads to variations in torque constant when the synchronous motor is driven, and of not being able to produce the maximum torque. Especially when the synchronous motor is rotated at such a high speed that weak field control is needed, it is difficult to feed a proper d-phase current when the initial magnetic pole position is inaccurate. This causes a lack of drive voltage to be applied to the synchronous motor, and consequently makes the control of the synchronous motor unstable. For these reasons, to control a synchronous motor, it is important to accurately detect the initial magnetic pole position when the synchronous motor is started. To obtain an accurate initial magnetic pole position, a sensor capable of detecting a magnetic pole position as an absolute value is used in some cases. In such a case, the sensor is sometimes aligned with the magnetic pole position when being mounted. Alternatively, to omit the alignment operation, a motor controller carries out a magnetic pole position detection process when the synchronous motor is started, for example.
Various methods of detecting a magnetic pole position have been proposed. For example, Japanese Patent No. 3408468 proposes a method of detecting a magnetic pole position by feeding a current to a predetermined excitation phase of a stator in a short period of time and repeating feeding of a current to a different excitation phase in a short period of time on the basis of the direction in which the rotor rotated at the previous current feeding.
Moreover, Japanese Patent No. 3805336 proposes a method of detecting a magnetic pole position in a synchronous motor having saliency by observing current feedbacks while changing an excitation phase by inputting a high-frequency voltage command.
Furthermore, Japanese Laid-open Patent Application Publication No. 2002-136174 proposes a method of performing sensorless rotation control on a synchronous motor when the synchronous motor is in steady operation, although this method is not only applicable to detection of a magnetic pole position at the time when the synchronous motor is started. According to the method described in Japanese Laid-open Patent Application Publication No. 2002-136174, the sensorless control is made possible by causing the phase of each voltage command to conform to the phase of the corresponding counter electromotive voltage induced in the synchronous motor, by using a phase locked loop (PLL) circuit.
According to the method described in above Japanese Patent No. 3408468, the rotor needs to be rotated minutely. Consequently, the accuracy in detecting the initial magnetic pole position is affected by the friction and inertia of the rotor. Therefore, in some cases, it is difficult to detect the position when the rotor is locked.
Moreover, according to the method described in above Japanese Patent No. 3805336, the value of each high-frequency voltage command to be input needs to be adjusted according to the degree of saliency of the synchronous motor. Accordingly, when the saliency is low, the accuracy in detecting an initial magnetic pole position is poor. In addition, filtering is performed by differentiating current feedbacks, and a magnetic pole position is detected on the basis of the magnitudes of the amplitudes. For this reason, detection using this method is likely to be affected by noise.
Further, the method described in above Japanese Laid-open Patent Application Publication No. 2002-136174 uses counter electromotive voltage. Accordingly, the rotor of the synchronous motor needs to be rotated. For this reason, when this method is employed to detect an initial magnetic pole position, the accuracy in detecting an initial magnetic pole position is affected by the friction and inertia of the rotor, as in the case of Japanese Patent No. 3408468 described above. As a result, it is difficult to detect the position when the rotor is locked, for example.